Topic: Multiple GPUs slow down hashrate (Read 678 times)

I bought a couple of HIS 280x to check out how it goes. I have 2 win7 boxes, one is 1.5 year old, one is 4.

With some struggle (had to buy new power supplies and a new case) I made the cards work on both of them at 730 khs. So i figured I'll add 2 more because i had 3 more open slots.

The older board does not recognize more than one card (updated bios and all of that) which is OK with me, even though it runs the card cooler and with no display artifacts.

The newer board (gigabyte z77x-ud5h, cpu i7-3770, 8 gigs ram) can run 3 cards at the same time without risers but the hashrates drop from 730 to 500 if I add more than 1 card. And I've been playing with cgminer settings quite a bit - no help. The power supply is a new Corsair RM1000, so it should be OK from this perspective.

I kind of think it's gotta be an issue with older motherboards but maybe I'm wrong. I was thinking of trying Linux (i've had a quad-boot system before)  but I'm a bit short on time, considering I'm not even sure it would make a difference. The other rout would be buying a new motherboard (and maybe RAM and processor as well in which case it kind of becomes a mission creep...). Three of my cards are identical (850 core) one runs hotter and slower, kind of opposite of what's expected (1000 core)...

Thanks in advance for any ideas.

prolom.